The present invention relates to a fast retraining method, a fast retraining apparatus, and a storage medium for storing a program of a fast retraining method, for implementing a high speed acquisition of synchronism in an xDSL transceiver such as an Asymmetric Digital Subscriber Line (ADSL) transceiver. In particular, the present invention relates to a technique for acquisition of synchronism at high speed in a simple version of an ADSL, i.e., an Universal ADSL, which does not need a splitter for separating high frequency components for the ADSL communication from low frequency components for usual telephone communication.
The ADSL transceiver is a unit for implementing a high speed data transmission by utilizing a conventional telephone circuit (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 10-303872, and xe2x80x9cSpecial Corner, Start to Count Down, ADSL for a Country of Lightxe2x80x9d in Nikkei Communication, Dec. 7, 1998). That is, while a telephone circuit uses a low frequency band of 300-3400 Hz, an ADSL transceiver utilizes a high frequency band of 25 KHz to 552 KHz (ITU-F 992.2 standard of 25 KHZ) or 25 KHz to 1.1 MHz (ITU-G. 992.1 standard). When a usual telephone circuit is being used, the frequency band is usually limited to the above-mentioned low frequency band (300 to 3400 Hz). However, even when the telephone circuit is being used, some of the control signals such as an off hook signal, an on hook signal, a polarity inverted signal which is generated when the telephone circuit is switched from a released state to a terminating state, a voltage change of a ring signal, etc., leak to the frequency band of 25 KHz or more used in ADSL communication.
In normal ADSL (the ADSL (G. dmt) of G.992.1 standard) communication, a frequency splitter is provided in the telephone circuit to remove the above-mentioned leakage signal.
In Universal ADSL communication, however, the frequency splitter is not provided in the telephone circuit so that the above-mentioned leakage signal is superimposed on the ADSL line to interrupt data communication.
That is, in Universal ADSL communication, every time a telephone instrument goes on hook or off hook during data communication by the ADSL transceiver, the data communication is stopped or an error is generated.
In general, when the ADSL transceiver detects an error generated on the telephone circuit, the ADSL transceiver is automatically reset to perform an initial training to restart the communication.
However, in Annex-C (ISDN Special Noise Removing Additional Recommendation) of the G.992.2 standard, the above-mentioned initial training takes a long time, i.e., about 20 seconds plus several seconds. Therefore, the leakage of a control signal from the telephone circuit causes significant damage to the ADSL data communication.
In view of the above problem, the G.992.2 standard defines a fast retraining sequence, whereby a faster data communication recovery can be realized. According to this fast retraining sequence, when an error is generated in a telephone circuit during data communication, the process enters into the fast retraining sequence, and then the ADSL transceiver checks the state of a telephone instrument which is provided outside of the ADSL transceiver. For example, when the telephone circuit is changed from about 48V in an open state (i.e., an on hook state of the telephone instrument) to about 6V when the telephone instrument is off hook so that the telephone circuit is terminated, the ADSL transceiver searches whether there is data indicating an off hook profile in a memory in the ADSL. transceiver. If there is no such profile, the fast retraining sequence is stopped, and after effecting an initial training of about 20 seconds plus several seconds, the data communication is started again. Thus, when there is no profile which matches the state of the telephone circuit, or when the ADSL transceiver fails to handshake with the other station within a predetermined time, the fast retraining sequence is stopped and the process transfers to the initial training. Here, the handshake means an exchange of a profile or an exchange of an S/N level between two transceivers or modems.
If, however, there is an off hook profile, the initial training is not carried out, and after the fast retraining sequence which takes only 5 to 6 seconds, the data communication is carried out.
In this way, when the state of an ADSL transceiver changes from an initial training to data communication, various parameters such as a parameter for an automatic gain control (AGC), a parameter for a phase locked loop (PLL), the depth of codes of a Reed-Solomon Block Coding for error correction, an on hook state, an off hook state, an open circuit voltage and a terminating voltage of the telephone circuit, etc., are stored as profiles (i.e., initial values). In the initial training, matching of these parameters is effected between a center and a user""s home by handshaking, and the matched parameters are stored in the memory in the ADSL transceiver.
Then, when the process changes to a fast retraining sequence, if there is a profile which matches the line characteristics after the state of the ADSL transceiver has been changed, the process does not change to the initial training but a data communication is carried out after executing the fast retraining sequence.
Also, only when there is no profile which matches the line characteristics after the state of the ADSL transceiver has been changed, the process proceeds to the initial training.
In this way, the data communication be started earlier.
When the above procedure is applied to an actual telephone circuit, however, the following problems arise.
In the control signals of a telephone circuit, there are signals such as dial pulses which disturb the operation of an ADSL transceiver. In the following, an example is described in which, when dial pulses from a telephone circuit are generated, it is difficult to effect the fast retraining sequence.
When a telephone instrument at a calling side goes off hook, noises are injected into the high frequency band for the ADSL communication which causes data errors. In response to the occurrence of these errors, the ADSL transceiver starts the fast retraining sequence. In response to the start of the fast retraining sequence, a handshaking between one ADSL transceiver and another ADSL transceiver is started. However, the telephone instrument continues to generate the dial pulses. Each time the dial pulse is generated, a noise is generated. Due to the noise, the handshaking, including profile exchange or S/N level exchange, fails. In the prior art, when handshaking failures are repeated for a predetermined number of times, it is deemed that the fast retraining sequence has failed so that the fast retraining sequence times out, and the process changes to an initial training which requires about 20 seconds plus several seconds. Therefore, the state of the ADSL transceiver cannot be expected to recover to the data communication status within a short time.
In addition, as is well known, each of the ten numbers is expressed by a number of dial pulses. Assuming that the number of pulses for a unit of time is 10 pulses per second, then the maximum time necessary to send ten digits is about 10 seconds, i.e., (10 pulses/10 pps)xc3x9710 digits.
It is possible to incorporate this maximum time as a waiting time into the fast retraining sequence, however, if the fast retraining sequence is so constructed, the timer will start to count the above-mentioned maximum time even by an on hook or off hook operation of a telephone instrument without generating any dial pulse, and consequently it will become impossible to xe2x80x9crecover to the data communication status within a short timexe2x80x9d which is the advantage of the fast retraining sequence. Further, when a local communication is effected, an out-of-town telephone exchange number is not necessary so that the sending of the dial pulses is performed within a short time, considerably shorter than the maximum necessary time. In this condition, if the waiting time is set to be the maximum time necessary to send ten digits, a problem arises in that the time wasted while waiting becomes too long.
Accordingly, an object of the present invention is to provide a fast retraining method in an xDSL transceiver, a fast retraining apparatus in an xDSL transceiver, and a storage medium for storing a program of a fast retraining method in an xDSL transceiver.
To attain the above object, there is provided, according to a first aspect of the present invention, a fast retraining method in an xDSL transceiver comprising the steps of: detecting an error in a telephone circuit connected to the xDSL transceiver; and changing the process in the xDSL transceiver to a fast retraining sequence when the cause of the error is an event other than a change from an on hook state to an off hook state of a telephone instrument connected to the telephone circuit.
According to the second aspect of the present invention, the above-mentioned event is a change from an off hook state to an on hook state of a telephone instrument connected to the telephone circuit.
According to the third aspect of the present invention, the event mentioned in the first aspect is an inversion of the polarity of a signal of the telephone circuit.
In the first to third aspects of the present invention, the fast retraining sequence is forced to be carried out regardless of whether or not a dial pulse is detected. Therefore, the time out of the fast retraining sequence due to the reception of a dial pulse does not occur, and consequently the recovery time of the xDSL data communication is shortened.
According to the fourth aspect of the present invention, there is provided a fast retraining method in an xDSL transceiver, in which, when an error is detected on a telephone circuit connected to the xDSL transceiver, and when the cause of the error is a change from an on hook state to an off hook state of a telephone instrument connected to the telephone circuit, a predetermined handshake waiting time is set. The handshake waiting time starts from the time of a change from an on hook state to an off hook state of a telephone instrument connected to the telephone circuit, and has a length longer than a dial pulse waiting time. The process in the xDSL transceiver changes to a fast retraining sequence when the handshake waiting time has passed without detecting a predetermined signal in the telephone circuit.
Since the fast retraining sequence is forcibly carried out when the handshake waiting time, which is set to be longer than the dial pulse waiting time, has passed, regardless of whether or not the dial pulse is detected, the fast retraining sequence does not time out due to dial pulse reception, and consequently the recovery time of the ADSL data communication is shortened.
According to the fifth aspect of the present invention, when a predetermined signal in the telephone circuit is detected within the above-mentioned handshake waiting time, the process changes to a fast retraining sequence.
Since, regardless of whether or not the dial pulse is detected, the fast retraining sequence is forcibly carried out when the predetermined signal is detected within the handshake waiting time, which is set to be longer than the dial pulse waiting time, the fast retraining sequence does not time out due to dial pulse reception, and consequently the recovery time of the xDSL data communication is shortened.
According to the sixth aspect of the present invention, when a dial pulse signal in the telephone circuit is detected within the above-mentioned handshake waiting time, the pulse width of the dial pulse signal is measured, and the handshake waiting time is updated by a product t=Nxc3x97p, where p is the dial pulse width, and N equals the maximum number of digits in the dial pulse signal multiplied by the maximum number of pulses per one digit. The process in the xDSL transceiver changes to a fast retraining sequence when the updated handshake waiting time has passed without detecting a predetermined signal in the telephone circuit.
In the sixth aspect of the present invention, when a dial pulse signal in the telephone circuit is detected within the handshake waiting time, the handshake waiting time is updated in accordance with the pulse width of the dial pulse signal and the maximum number of digits in the dial pulse signal. The fast retraining sequence is forcibly carried out when the updated handshake are waiting time times out. Since the updated handshake waiting time is longer than the maximum dial pulse duration, the process does not change to the fast retraining sequence during the dial pulses duration, and consequently, during the dial pulse duration, the process does not fail to change to the fast retraining sequence so that the recovery time of the ADSL data communication is shortened.
According to the seventh aspect of the present invention, the process in the xDSL transceiver changes to a fast retraining sequence when a predetermined signal in the telephone circuit is detected within the updated handshake waiting time which is defined in the sixth aspect of the invention.
In the seventh aspect of the present invention, since the fast retraining sequence is forcibly carried out when the predetermined signal in the telephone circuit is detected within the updated handshake waiting time which is longer than the dial pulse duration, the process does not change to the fast retraining sequence during the dial pulses duration, and consequently the recovery time of the ADSL data communication is shortened.
According to the eighth aspect of the present invention, when a dial pulse in the telephone circuit is detected within the handshake waiting time defined in the above-mentioned seventh aspect of the present invention, and when it is determined that the dial pulse comes through a telephone office other than NTT, the value N is updated to N2=N+xcex1, and the handshake waiting time is updated by a calculated value t2=N2xc3x97p. Then, when the predetermined signal is detected on the telephone circuit within the updated handshake waiting time, the fast retraining sequence is carried out.
In this way, since the handshake waiting time is updated to an optimum value depending on whether the dial pulse comes through NTT or another telephone office, an optimum recovery time of an ADSL data communication is realized.
In the fourth to eighth aspects of the present invention, the predetermined signal in the telephone circuit is one of a signal indicating that the polarity of a signal in the telephone circuit has been changed, a signal indicating a change from an on hook state to an telephone off hook state of a telephone instrument connected to the telephone circuit, a push button dial signal from the telephone instrument connected to the telephone circuit, a ring-back tone from the telephone instrument connected to the telephone circuit, or busy tone from the telephone instrument connected to the telephone circuit.
Further, according to the present invention, there are provided an apparatus and a storage medium for storing a program which performs the above-mentioned fast retraining sequence.